Drilling the Crust – Mantle transition zone in the Oman ophiolite – the formation of massive dunites.

The formation of oceanic crust at mid-ocean ridges is one of the dominant processes in the chemical differentiation of our planet. Oceanic crust formed at fast-spreading ridges exhibits a relatively uniform seismic stratigraphy and is regarded as layered and relatively homogeneous. Because of the lack of in-situ exposures at the base of recent oceanic crust, existing models on the geodynamics of the deep processes during crustal accretion have never been tested directly using natural samples. The ICDP Oman Drilling Project penetrated at two sites the crust/mantle boundary in the Oman ophiolite, the best analogue for fast-spreading crust on land (drill cores CM1, CM2). We started a study investigating a continuing and densely spatial resolved sample set of both drill cores in order to shed light on the nature of this poorly understood zone at the base of the Oman paleocrust. The drill cores CM1 and CM2 cover the upper mantle harzburgites at the bottom, followed by a 90 m thick massive dunite layer with layered gabbros on top. Ni and Mg# in olivine as well as Cr#, Mg# and trace elements in chrome spinel were analyzed by EPMA and fs-LA-ICP-MS. The data reveals a homogeneous harzburgitic upper mantle composition and a dunite section showing decreasing Mg#, implying an increase in differentiation towards the top. We conclude that the zone of massive dunite was formed as a first cumulative crystallization event of a mantle-derived, primitive MORB melt, while the residual melt was fed into the stockwork system of the layered gabbros.